Arthritis, a common problem with many people, is a wearing or loss of the cartilage at any joint. The wearing or thinning cartilage can lead to inflammation, pain, loss of motion, and deformity. Arthritis can affect any joint including the wrist joint which contains numerous bones called carpal bones (as seen in FIG. 1). Fusing joint is a common orthopaedic surgical solution used to address arthritis and joint instability. In the wrist, there are two common forms of arthritis named specifically scaphoid nonunion advanced collapse (SNAC) and scaphoid lunate advanced collapse (SLAC). These types of arthritis result in a predictable progression of arthritis that involves the carpal bones of the wrist. One surgical procedure to address the arthritis is to perform a scaphoid excision and limited intercarpal fusion. Currently the most common form of intercarpal fusion involves fusion of the lunate-capitate-hamate-triquetral carpal bones. This is also referred to as the four-corner fusion. The four corner fusion fuses together the four carpal bones by various fusion or fixation methods or techniques. These fixation techniques include K-wires, headless screws, circular plates, and memory staples.
The problem with K-wires is they are unable to provide compression across the fusion plane, have limited fixation strength, are less rigid, and can lead to pin tract infections which can often result in early removal. Compression across a fusion plane is beneficial as it increases fusion rate and time to fusion. In addition, they often lead to secondary surgical procedures to remove the K-wires if they were buried during the index procedure.
Headless screws are technically challenging to use, usually provide less optimal compression across the fusion plane, and can lead to ulnar cutaneous nerve injuries. Further, when using a capitolunate headless screw one must choose the antegrade or retrograde position. In order to the insert the screw antegrade, one has to penetrate and create a hold in healthy viable cartilage of the lunate bone. Furthermore, the retrograde option is technically difficult, suboptimal in terms of screw purchase into the lunate, and the compression across the fusion plane is not perpendicular.
As seen in FIG. 2A, circular plates 21 have been used to assist in the four-corner fusion. The problem with the rather large circular plates 21 include: (1) nonunion or loose hardware; (2) hardware impingement; (3) post-operative pain; (4) low patient satisfaction; (5) limited pain-free patients; and (6) the plate alignment and installation procedure are technically difficult to achieve consistent success. The two most significant of these complications is the nonunion rate and hardware impingement. Reasons for the nonunion include the fact that these circular plates have limited ability to compress across the fusion planes. Further, although the current circular plate systems are designed to create a depression on the dorsal carpal bones through a reaming process, the plate must still partially rest on the dorsal lunate. This part of the dorsal lunate normally articulates with the dorsal distal radius at the upper ranges of wrist extension. Thus, these plates are prone to inherently impinge against the dorsal radius causing pain, decreased pain satisfaction, and at times necessitating plate removal.
As seen in FIG. 2B, memory stapes 22 provide an alternative to the circular plates or K-wires. Staples are also known to have inherent problems. Staples 22 can be difficult to insert and immediately start compressing when they touch the bone or when they are released from the inserting tool. Some staples require a secondary heating process to compress the staples which can cause bone necrosis and decrease fusion rates. Further, the tines on the staples do not provide a rigid fixation like screws, the staples can back out of the bone and they are not as strong as titanium (yield strength of 690 Mpa vs. 1000 Mpa for screws).
When attempting to fuse two bones, forcing compression of the bones against each other is a critical factor that determines if the bones will fuse or not. Ideally, one wants the compression force to be as perpendicular to the bone surfaces being fused. An additional problem with staples is that compression across the fusion is non-linear as the compression is greater at the end of the staples and less at the bridge. The force vector of the compression is therefore not perpendicular to the fusion
In addition to four corner fusions for wrist arthritis, two other forms of intercarpal fusion options exist. Three bone fusions consist of fusing the lunate, capitate, and hamate. Two bone fusion consists of fusing the lunate and capitate. With these two options, the triquetrum can either be removed or left in place but just not part of the fusion. In order to perform these two types of fusions, current techniques use similar constructs of K-wires, headless screws, or staples as described above for four corner fusions. There are no specific circular plates designed for two or three bone fusions.
In all three fusion options (2, 3, or 4 bone), the lunate needs to be fused with the capitate. The problem with performing a capitolunate fusion with screws is that the bones are very co-linear and there is not a good way to insert a screw across the fusion plane. Further, since ideal fusion stems from compression perpendicular to the fusion plane, access to and alignment of the perpendicular axis of the fusion plane is a significant challenge. When using a plate construct to fuse the lunate to the capitate, prominent dorsal hardware on the lunate presents a concern and the technique is difficult to reproduce.
Screw fixation of capitolunate can provide greater linear compression and more rigid fixation than stapes. However, inserting these screws can be a technical challenge for surgeons into these two carpal bones due to their size, shape and location. Two current techniques in inserting these screws freehand include: (1) antegrade; and (2) retrograde. Both techniques are difficult to perform, not ideal in angle or purchase of the bone, and not easily reproducible.
Unfortunately, screws are also not an optimal solution as a stand-alone device or method for capitolunate fusions. As seen in FIGS. 3A and 3B, the screw or screws 31, 32 are inserted into the main part of the capitate and lunate bones to provide optimal fusion. However, as seen in FIG. 4, the alignment 41 of the capitate and lunate bones relative to each other is difficult to obtain an optimal insertion procedure. In the antegrade method (see FIG. 3A), screw 31 insertion requires significant release to allow hyperflexion of the wrist. The procedure also leaves holes in good lunate cartilage. Sometimes, proximal screw prominence from migration or settling can be a problem causing damage to distal radius cartilage damage and requiring a second surgery to remove the screw. In the retrograde method (see FIG. 3B), the dorsal capitate technique is used which is technically difficult for surgeons to perform as the screw 32 insertion is not perpendicular to the fusion plate, it does not provide optimal lunate purchase, requires an intercarpal technique, is more difficult than the antegrade procedure, can lead to neurovascular injury; and can cause damage to carpometacarpal joints.
When placing a screw antegrade from the lunate to the capitate, two challenges are encountered. First, in order to get a screw as perpendicular to a standard fusion plane which would be the best compression and rigid fixation, one needs to perform significant release of the wrist joint in order to achieve extreme wrist flexion. In many patients with wrist arthritis, they already have significant wrist contractures making achieving this wrist flexion even more difficult. Secondly, in order to get this perpendicular path across the fusion plane, one has to drill and make a hole in the lunate cartilage. Again, this is not ideal as after the procedure the “new” wrist joint will be wholly comprised on this radio-lunate articulation.
Placing the screw in a retrograde fashion would avoid this last issue. However, because of the anatomic relationship of the lunate-capitate-third metacarpal, insertion of the screw is very difficult. These bones are co-linear and tightly bond. There is no edge or location that allows a screw to be inserted that is perpendicular to the fusion. Currently, surgeons utilizing this technique will usually place the screw(s) somewhat oblique to the fusion.
Therefore, what is needed is a new technique and device or devices enabling use of a screw to fuse the capitate and lunate bones which is generally perpendicular to the fusion plane. Ideally the device would not rest on the dorsal surface of the lunate so it wouldn't impinge on the distal radius. Variation of the device should allow for performing a 2, 3, or 4 bone fusion.